The true position accuracy of mechanical features of a molded article is determined by accuracy of associated features of a mold used to produce the article. The true position accuracy of the mold features is, in turn, determined by the accuracy of fabrication of the mold components, the accumulation or stack-up of tolerances of the various components comprising the mold, the clearance gaps required to enable the mold to be assembled and disassembled, the clearance gaps required to enable moving parts of the mold to move freely, the accuracy with which the two halves of the mold are positioned relative to each other, the movement of the mold components that results from forces imparted to the mold during the molding process, the deformations of the mold resulting from forces imparted to the mold during the molding process, and changes that occur to mold components as a result of wear and tear over the useful service life of the mold. The tolerance limits to the true position accuracy of features of the molded article, which result from the sum of the foregoing factors, determine the suitability or ability of a particular molding process to produce a particular article capable of satisfying specified fit and function requirements. These limitations apply across the entire range of molding technology including injection molding, compression molding, transfer molding, ceramic molding, metal molding, sintering, or glass molding, for example.
There is a continuing need to produce molded parts with accuracy requirements exceeding the cumulative accuracy permitted by the limiting factors set forth above. As mold fabrication techniques have improved, the tolerances achievable via molding processes have accordingly improved, enabling manufacture of molded articles having accuracies previously not achievable. However, there are inherent limits to what can be achieved by improving the precision in execution of conventional molding techniques and with conventional mold tooling architectures.
Molded subassemblies are rapidly displacing subassemblies formed out of discrete components. For example, conventional optical subassemblies used in fiber optic transceivers typically have multiple structural elements including expensive discrete glass lens arrays. Requirements of individual component costs and manual assembly cause such subassemblies to be relatively very expensive, currently on the order of about $40.00 per completed subassembly. The assignee of the present invention has developed injection molding processes which enable production of one-piece molded polymeric optical components for use in fiber optic transceivers at much lower unit costs, currently on the order of about $4.00 per completed component. Precision molded optical subassemblies formed of optical grade thermoplastic polymers find practical use not only in fiber optic transceivers but also in fiber optic connectors, cameras in cell phones and the like, sensors in printers and scanners, and biomedical devices, for example.
A typical injection molding process includes steps of bringing two complementary mold halves holding die inserts with features defining an article to be molded into a close facing proximity, injecting e.g., a thermally plasticized polymer material as an amorphous mass into a molding volume or space between the die halves; applying sufficient pressure to the plasticized polymer to cause it to conform to the features of the molding volume defined by the die halves; allowing the material to cool in the mold to cause the material to resume a solid phase (“freeze”); moving the die halves apart; and, removing the molded article. The molding process cycle may then be repeated. Evidently, there are many tolerances associated with injection molding, particularly with regard to maintaining accuracy of alignment of the moving die half relative to the fixed die half. Mechanical alignment tolerances include three positional dimensions (x, y and z), as well as rotational and tilt dimensions. Mold tolerances are additionally affected by changes in temperatures and pressures associated with the molding process as well as mechanical clearances and wear, as noted above.
Molding machines typically include massive frames and guiding structures and features, as well as temperature and pressure control systems, in order to regulate, control and hopefully minimize molding process tolerances. Yet, as dimensional requirements for molded articles approach the micron range, the conventional techniques for controlling tolerances have proven inadequate. Control of molding dimensions to a tolerance of about ±one micron has heretofore proven illusive, if not practically impossible. Yet such tolerance is needed in order to provide molded optical components for plural-lens single-mode optical fiber applications, for example.
Various tolerances associated with thermoplastic injection molding may be understood by considering the molding environment. FIG. 1 illustrates a conventional thermoplastic injection molding machine 10 that is fitted with a conventional mold. At a high level the molding machine 10 essentially includes a massive base or frame 12, a fixed platen 14 directly secured to the base 12, a fixed mold-half 16 secured to the fixed platen 14, a moveable platen 18, a moveable mold-half 20 secured to the moveable platen 18, a platen actuator 22 secured to the base 12 for moving the moveable platen 18 toward and away from the fixed platen 14 along leader pins 24, a hopper 26 for holding a supply of thermoplastic pellets, a plasticizer-injector 28 for plasticizing a quantity of thermoplastic pellets and for injecting a plastic-state amorphous mass via a conduit 30 through the fixed platen 14 and fixed mold-half 16 and into a molding volume 32 defined when the moveable mold-half 20 is forced to close against the fixed mold-half 16 by the actuator 22. Force equivalent to 20 to 250 tons, more or less depending upon the molding machine, may be exerted by the actuator 22 against the moveable platen 18 and moveable mold-half 20 during the mold-closing operation. FIG. 1 illustrates the molding machine in a mold-open position. Details such as heating/cooling supplies and conduits, automatic picker-gripper tooling for engaging and removing each molded article from the mold following molding, and mold machine controls, are not shown in FIG. 1 but would be present in practical embodiments of automatic molding apparatus, as is well understood by those skilled in the art.
FIG. 2 illustrates a mold-half 20 for molding thermoplastic precision optical lens arrays in accordance with the prior art. This conventional mold-half 20 includes a number of alignment features and components. To begin with, the leader pins 24 guide the moveable mold-half 20 relative to the fixed mold-half 16 with a tolerance of ±75 microns due to lubricant thicknesses, etc. Zero-degree interlocks 34 engage complementary features of the fixed-mold half 16 and reduce relative positional tolerance between mold halves to about ±12 microns. A cavity block 40 is mounted to each mold half. According to current industry practice, locational precision of the cavity block 40 relative to the mold-half 20 is established by a precision fit with sufficient tolerances or gaps to enable insertion and removal of the block 40 from the mold-half cavity.
Cavity-block zero-degree interlock pins 42 register the cavity blocks together at mold closure with a tolerance of about ±12 microns. Angled taper locks 44 projecting from the moveable cavity block 40 mate with complementary angled recesses of the fixed cavity block (not shown in FIG. 2) and reduce closure tolerance to about ±3 microns, establishing the smallest molding tolerance achievable with contemporary molding techniques of the prior art. A mold insert 46 defining structural features to be formed in the molded article is precisely fitted into an opening of the cavity block 40 and locked in place. The positional accuracy of the mold insert 46 relative to the cavity block 40 is limited by the precision of fit achievable in the particular molding operation. In the present example of a molded component with two precision optical lens surfaces, diamond-ground precision optical lens pins 48 are installed in the mold insert with a precision limited by the particular fit.
In order to form a precision optical lens molded article in accordance with conventional practice, the mold halves 16 and 20 are assembled and installed on the molding machine 10. A test article is then molded and removed from the machine and carefully measured under a microscope, magnifying optical comparator, or other suitable tolerance measuring apparatus or device to determine dimensional errors and tolerances. Correction calculations are then carried out based on measured errors. At least one of the mold halves 16 and 20 is then removed from the machine, disassembled, dimensionally adjusted to reduce the measured errors in the test article and reassembled. Dimensional adjustments may be carried out by machining to remove mold material and/or plating or other deposition to build up mold material. The testing/adjustment process is repeated until an article is molded having acceptable dimensional/optical tolerances and qualities. Obviously, this mold setup procedure is very time consuming. Additionally, during a production run, molded articles are selected and manually checked to be sure that the molding process remains within tolerance. If articles are found to be out of tolerance, production is stopped and another setup procedure of the type described above is undertaken to correct the out-of-tolerance condition. Also, even though the mold halves 16 and 20 are regulated at precise temperatures and pressures during the molding process, control of molding tolerance at a ±one micron level of accuracy of the molded precision article has not been possible with contemporary techniques.
The prior art suggests several techniques for adjusting mold dimensions without requiring removal, disassembly, reassembly and reinstallation of a mold set. In U.S. Pat. No. 5,512,221 to Maus et al., entitled “Lens Thickness Adjustment Method and Apparatus in a Thermoplastic Injection Mold for Opthalmic Finished Spectacle Lenses” a wedge block operated by a manually rotated adjustment knob external to the mold provided a mold-half and mold cavity adjustment to change molded spectacle lens thickness without requiring disassembly of the mold. A slightly different approach using a worm gear mechanism in lieu of a wedge block to change molded spectacle lens thickness is described in U.S. Pat. No. 5,792,392 to Maus et al., entitled: “Lens Thickness Adjustment in Plastic Injection Mold”. These patents concerned controlling spectacle lens thicknesses in the millimeter range, as opposed to the micron range, and as described would not provide sufficiently accurate mold tolerance control to achieve tolerance control in the ±one micron range, due to mechanical tolerances and hysteresis associated with the mechanical components employed in the teachings of these patents to alter the mold thickness.
When thermoplastic material changes from a thermoplastic state to a solid state at the end of the molding process, the material typically shrinks slightly. A variety of techniques are known in the art to compensate for shrinkage. One approach is described in Japanese Published Patent Application 61-66623 published on May 4, 1986. This approach measures mold volume indirectly with a variable resistance sensor coupled between the fixed and moveable mold halves and automatically controls mold cavity dimension to achieve a predetermined article thickness. Again, this approach does not appear to describe a molding process having sufficient accuracy to achieve molding tolerance control in the ±one micron range.
Active alignment techniques are employed in the optical fiber splicing art in order to maximize light transmission at a fiber splice. In the active technique, light energy is launched into one fiber and its amplitude is measured through the other fiber. The fiber ends are automatically manipulated and spatially/axially adjusted in a manner to produce maximum transmission of light energy thereby denoting axial alignment of the ends. Then, the abutting fiber ends can be joined together by welding or bonding. While active alignment techniques have been employed in optical fiber splicing, they have not heretofore been applied to control mold alignment in a molding process for molding precision articles and components in order to achieve accuracy in the micron range.
A hitherto unsolved need has arisen to provide methods and apparatus enabling precision molding of thermoplastic optical articles having dimensional tolerances controlled to an approximate ±one micron range of accuracy.